I have the Sol and have used it for a couple of years.
I have no complaints about the legibility of the screen, in the dark or in poor vis. The backlight is not nuclear bright like the Cobalt looks to be, but it's fine.
The HRM is fun and I try to remember to put the darn strap on before I suit up. I use the heart rate data to help assess my workload level of relaxation. It's a gimmick, IMHO, but fun to have.
The first thing I did was set the computer to ignore the heart rate data when it does its calculations. My Sol's numbers then are just like my buddy's Uwatec computer. I am not sold on the advantages of folding heart rate into the calculations. The deco math black box is already pretty opaque, is an additional complication really helping? The manufacturer hasn't published any sort of giant study that I am aware of.
The Sol battery is easy to change, I hear. Haven't had to do it yet.
If I was buying a new computer today, I'd be looking at the Luna/Sol and the Mares, because I learned that I want an absolutely huge screen. There is a very good chance I'd get the Sol again.